A voltage regulator is a device that provides a regulated voltage that remains substantially constant as load current and supply voltage change. Typically, a voltage regulator includes a large pass transistor to pass current into the load, and the regulator is stabilized externally using a large external capacitor, such as a 0.1 μF to 1 μF capacitor, and a small on-chip metal resistance. However, the large external capacitor increases the parts count, increases the cost of the voltage regulator and dramatically reduces the regulator's bandwidth of operation.
Instead of using a large external capacitor, the voltage regulator may be stabilized by connecting a Miller capacitor from the gate of the large pass transistor to the drain of the large pass transistor. While the Miller capacitor provides a compact method of stabilizing the voltage regulator, it passes high frequencies and therefore significantly reduces the Power Supply Rejection Ratio (PSRR) of the regulator. Further, the voltage regulator stabilized by the Miller capacitor has a very low frequency response, approximately 10 KHz–100 KHz. Accordingly, in systems that operate above 100 KHz, the voltage regulator may require more than one clock cycle to settle the regulated voltage to its desired value each time the load current changes. Requiring more than one clock cycle to settle the regulated voltage introduces errors into systems where the regulator is used as a voltage reference for signal processing blocks and is therefore undesirable, especially in wireless communications environments. It should be noted that this technique also reduces the regulator's bandwidth of operation.
Thus, there remains a need for a voltage regulator that is stabilized by an on-chip capacitor and that has both a fast settling time and a high PSRR.